1. Field of the Invention
The present invention relates to a metal halide lamp used as a light source of a liquid crystal projector and the like.
2. Description of the Prior Art
Recently, liquid crystal projectors or the like are known as means for enlarging and projecting an image of characters, figures, etc. Since a certain optical output is required for such an image projector, a metal halide lamp, which has a high luminous efficacy, is widely used as a light source for the image projector in general. In this kind of metal halide lamp, as disclosed in Japanese Unexamined Patent Publication No. 3-219546 for example, an iodide of Nd, Dy, and Cs has been generally used as a halide to be filled in an arc tube.
The lamp in which the iodide of Nd, Dy and Cs is filled (hereinafter described as Dy--Nd--Cs--I series lamp), as disclosed in Japanese Unexamined Patent Publication No. 3-219546, has an excellent luminous efficacy, however, it has the disadvantage that devitrification occurs in an early stage of its lifetime, particularly because of high reactivity between neodymium iodide (NdI.sub.3) and silica glass of the arc tube. The lack of transparency decreases the intensity of light beam, reduces the luminous intensity and disperses the light beam, resulting in nonuniform illumination intensity and reduced brightness on a screen of the liquid crystal projector. Specifically, the drawback of the Dy--Nd--Cs--I lamp as use as the light source of a liquid crystal projector is that the lifetime of the liquid crystal projector is short.
Recently, from the viewpoint of saving energy, there is a demand for a light source having a higher luminous efficacy than that of the Dy--Nd--Cs lamp. "Luminous characteristics of metal halide lamp containing rare earth halide" (Journal of Lighting Society, vol. 65 No. 10, 1981, page 17) discloses that a light source having a high luminous efficacy can be obtained by combining a rare earth halide with a halide of Tl or In. However, the low correlated color temperature of the light source disclosed therein is unsuitable for a light source of such things as a liquid crystal projector. An example of a light source disclosed therein is a metal halide lamp in which InI and TmI.sub.3 are filled, and a correlated color temperature estimated from the relative spectral distribution diagram disclosed therein is about 4500 K. On the other hand, a white color reference for an image projector such as liquid crystal projectors is about 9000 K.